The London-based rights organization called the conditions at ADX "unacceptably harsh" and said they "breach international standards" for the humane treatment of prisoners.

Prisoners are deprived "meaningful social interaction" for years on end and the failure to provide "suitable, daily outdoor exercise" falls short of UN minimum rules for the treatment of prisoners, the report said.

Robert Hood, a warden at ADX Florence from 2002 to 2005, has described a jail as "a clean version of hell." "It's far much worse than death," he has told American media.

The description resonates with Jamie Fellner, a specialist in US prisons who works for Human Rights Watch in New York who has visited twice.

"It's a high-tech version of hell," she told AFP.

"ADX is a modern, clean, quiet, well-run facility, you walk down the hall, it's all very shiny," she said.

All prisoners she met were strip searched before and after leaving their cell, and kept behind plexiglass the entire time.

"It is hard to communicate the intensity and relentlessness of the supervision of these people," Fellner said.

Tsarnaev will be subject to the same "special administrative measures" or SAMS that have governed his contact with the outside world since July 2013.

He will be consigned to the special "H" unit -- one of the most restricted wings of the prison.

Besides his lawyers, he can receive only limited visits from his closest relatives, again held behind transparent glass.

He will be allowed two telephone calls a month of 15 minutes each, a former federal prisons official Mark Bezy told his trial.

His mail will be restricted and systematically opened and read.

In 2012, Craig Haney, a University of California psychology professor, testified before Congress that "a shockingly high percentage" of the prisoners in solitary confinement were mentally ill, "often profoundly so."

A law suit "Cunningham v. Federal Bureau of Prisons" filed in 2012 also offers a grim snapshot of life at ADX.

It alleged that "many" prisoners "interminably wail, scream and bang on the walls of their cells."

Some mutilate themselves with razors, shards of glass, sharpened chicken bones, writing utensils and whatever they can obtain.

"A number swallow razor blades, nail clippers, parts of radios and televisions, broken glass, and other dangerous objects," it said.

Others converse with voices in their head, or spread feces and human waste throughout their cells, throw it at wardens and "create health hazards."

"Suicide attempts are common; many have been successful," the complaint said